I think Leo Fender slanted the bridge pick up because it looks really cool.

Seriously, I'm sure Leo Fender did it to somehow save money. That is why he did everything else. And that's not a knock. The low manufacturing cost is one of the reasons the Stratocaster is a superior design ot the Les Paul. Back in the day it probably cost 1/4 to 1/3 to produce a Strat vs. a Les Paul. And we could sit here and legitimately argue which is the better guitar.

The Les Paul is Greek Revival Architecture. The Strat is Frank Lloyd Wright.